Hibernoma with broad bone erosions

Clinical History

An incidental and indolent buttock tumour was found in a 65-year-old woman after perform a Computed Tomography (CT) for breast cancer staging. We did not have previous pelvic imaging studies of the patient.

Imaging Findings

The initial CT showed a lipomatous soft tissue mass in the left gluteal muscles in contact with the iliac bone, causing broad and deep bony erosion of sclerotic borders, without cortical rupture, reflecting low radiographic aggressiveness and probably long-term evolution.
Magnetic Resonance Imaging (MRI) illustrates a thin peripheral capsule with superior disruption in communication with subcutaneous fat tissue (SFT) of the dorsal region. The tumour presents heterogeneous signal intensity similar but not identical to fat tissue in T1-weighted sequences with patchy areas of slightly lower signal and absence of a complete fat suppression in SPIR sequences. The tumour showed heterogeneous uptake after intravenous contrast administration which reflects internal vascularisation and also showed thin internal septa. No signal alterations in the bone marrow of the pelvic bones were shown.

Discussion

Hibernomas are soft tissue tumours of brown fat with radiographic benign behaviour. They usually have well-defined margins and/or a peripheral capsule, and they are characterised by a typical slightly lower heterogeneous signal intensity compared with mature fat tissue in T1 sequences. Occasionally the absence of complete suppression in STIR sequences in relation to associated fibrovascular tissue component is found. [1] After contrast administration, they present intense internal enhancement of the brown fat component, showing fine septa (<2 mm) and internal vessels. [2]

Its different radiological behaviour in comparison to benign lipomas may lead a differential diagnosis to other malignant lipomatous tumours such as well differentiated liposarcomas. The absence of solid poles, thick septa (> 2 mm) or other local aggressive imaging features helps us to rule out this diagnostic possibility. [3]

It is very unusual to find associated extensive erosions when the tumour is placed to the bone, with only one previous reported case. [4] The absence of pain or increased local temperature which are frequently associated and the slightly older age of the patient than usual makes this case very atypical.

The definitive diagnosis was provided by US- guided biopsy [5] and the pathology report from the patient revealed a lipomatous lesion with brown adipocytes that resemble brown fat, positive with S100 stain and streaks of fibrovascular tissue.

In several cases the biopsy does not exclude hibernoma differentiation in well-differentiated liposarcomas and that is why complete excision is indicated without reported local recurrence. [6]

In our patient, the tumour was completely resected and the final histopathology study of the specimen did not find malignant cells, supporting the diagnosis of hibernoma like the biopsy report.

The patient received systemic and surgical curative treatment for her locally advanced breast cancer and remains asymptomatic until the present without recurrence signs in two year follow-up imaging studies.

Differential Diagnosis List

Final Diagnosis

Hibernoma

URL:	https://eurorad.org/case/15509
DOI:	10.1594/EURORAD/CASE.15509
ISSN:	1563-4086